Thermionic ionization detector with radiant heater assembly

ABSTRACT

Method and apparatus for the thermionic ionization detection of one or more particular constituent components of a sample that is present in a fluid mixture flow provided to a thermionic source. Radiant energy is directed from a radiant energy source to the thermionic source. The radiant energy is sufficient to effect an elevated temperature in the thermionic source. The heated thermionic source causes ionization of the constituent components by means of an ionization process in which electrical charge is transferred from the thermionic source and converted into gas phase ion species. The current of gaseous ions is collected and measured at a collector electrode adjacent to the thermionic source

FIELD OF THE INVENTION

The present invention relates generally to thermionic ionizationdetectors, and in particular to a thermionic ionization detector havingan improved heater assembly for effecting a predetermined elevatedtemperature in a thermionic source.

BACKGROUND OF THE INVENTION

Thermionic ionization detectors are used in the field of chromatographyfor the detection of specific constituent components of a sample that ispresent in a fluid stream. Such detectors usually include a thermionicsource having a surface impregnated with an alkali metal compound so asto make the detector specifically sensitive to a halogen, nitrogen, orphosphorus compounds. An electrical heating current, carried by aresistive heating wire embedded in the thermionic source, heats thethermionic source. Certain sample compounds, or their decompositionproducts, extract the electrical charge from the hot thermionic surfaceof the source. Ions form on the surface of the thermionic source andmigrate through a fluid stream flowing past the thermionic source to acollector electrode. The resulting ion current is collected at thecollector electrode. An electronic current-measuring circuit, such as anelectrometer, measures the ion current arriving at the collectorelectrode.

The ionization mechanism in these thermionic detectors is believed bysome practitioners to be a surface ionization process rather than a gasphase process. (See, for example, Patterson, Journal of Chromatography,Vol. 167, p. 381, 1978.) Prior art thermionic detection techniques havetherefore attempted to improve the construction and performance of thethermionic source. For example, U.S. Pat. No. 2,795,716 discloses adetector featuring a source in the form of a cylindrical alumina ceramiccore upon which is wound a heater coil; U.S. Pat. No. 3,852,037discloses the deposition of a material in the form of a bead onto anelectrical heating wire to form the source.

Accordingly, FIGS. 1-A through 1-D represent the typical shapes andconfigurations of the ion collector (C), thermionic source (S), andsample inlet (I) in commercially available detectors. In FIGS. 1-A and1-C, the source (S) is formed as an alkali-glass bead fused on a heatingwire in the shape of a loop. In FIG. 1-B, the source (S) includes aheater wire wrapped on a ceramic core having an alkali-glass materialfused over the outer surface to form a bead. In FIG. 1-D, the source (S)includes a sub-layer coating of ceramic cement and a non-corrosive,metallic compound additive, and a surface layer of a mixture of ceramiccement and an alkali metal compound additive, that are molded about aloop of heating wire to form a solid cylindrical bead. A conventionalthermionic source is thus designed as a solid element that is positionedwithin the fluid stream. When the fluid stream is flowing, the majorityof the contact of the fluid stream with the thermionic source occurs onthe leading (upstream) portion of the exterior of the thermionic source.

The sensitivity of conventional thermionic ionization detectors isaffected by changes in the temperature of the thermionic source (S).Because the fluid stream tends to cools the surface of the thermionicsource, variations in the flow rate and direction of the fluid streamover the surface of the thermionic source reduce the stability andcontrollability of the temperature of the thermionic source. As aresult, the accuracy and the sensitivity of the detector is less thandesired.

Moreover, the alkali-metal compounds in the thermionic source arecorrosive to the metallic heating wire that is typically employed toheat the alkali compounds; some samples include chemical components thatare corrosive as well. Corrosion of the heating wire is known to causedetector failure and accordingly conventional approaches have attemptedto decrease the exposure of the wire to corrosion. One such approachincludes coating the metallic heating wire with a sub-layer comprised ofnon-corrosive ceramic material or a mixture of ceramic material and aninorganic, electrically conductive and non-corrosive chemical additive.See, for example, U.S. Patent No. 4,524,047. However, the success ofthis approach depends upon the integrity of the coating; the developmentof voids or cracks in the coating during manufacture or operation canlead to corrosion.

SUMMARY OF THE INVENTION

This invention provides a method and apparatus for an improvedthermionic ionization detection of one or more particular constituentcomponents of a sample that is present in a fluid mixture.

In a preferred embodiment of the present invention, a thermionicionization detector may be constructed for detecting the presence of aconstituent component of a sample in a first fluid. The detectorincludes a thermionic source having a matrix including an alkali metalcompound which is capable of ionization of the constituent component toproduce an ion current when operated at an elevated temperature. Aradiant energy source is provided for the emission of radiant energy.The radiant energy source is positioned relative to the thermionicsource such that the emitted radiant energy is sufficient to effect theelevated temperature in the thermionic source. A fluid mixing structureprovides a fluid mixture flow that includes the first fluid to thethermionic source and a collector electrode is provided for receivingthe ion current.

In comparison to the conventional metallic heating wire that is embeddedin an alkali-metal bead, the contemplated radiant energy source providesradiant (rather than conductive) heating of the thermionic source, suchthat a predetermined temperature may be achieved more accurately andconsistently in the thermionic source. As a result, the ionizationprocess is improved and the detector output signal is less susceptibleto the baseline drift and instability experienced by thermionicionization detectors of the prior art. A thermionic ionization detectorconstructed according to the present invention also benefits from theisolation of the radiant energy source from the corrosive effects of thealkali metal compound and the fluid mixture by means of a fluid-tightenvelope that surrounds the radiant energy source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D are simplified schematic illustrations that represent theconstruction of thermionic sources employed in thermionic ionizationdetectors found in the prior art.

FIG. 2 is a simplified schematic illustration of a thermionic ionizationdetector constructed according to the present invention.

FIG. 3 is a cross-sectional illustration of the thermionic ionizationdetector of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will find useful application in a variety of fluidanalysis systems that benefit from the detection of one or moreparticular constituent components of a sample present in a fluidmixture. Such systems are commonly employed in a wide variety ofapplications, such as sample analysis or purification, chemicalanalysis, clinical assay, environmental monitoring or sensing,industrial processing, and water purification. Further examples that areparticularly benefited by use of the present invention includesupercritical fluid chromatography and high pressure gas chromatography.

FIG. 2 shows a schematic illustration of the electronic and mechanicalcomponents of a first preferred embodiment of a thermionic ionizationdetector (D) constructed to include a flow-though thermionic source(hereinafter, simply termed as the thermionic source (11)), a radiantheater assembly (12), and a collector electrode (13) according to thepresent invention. The cylindrically-shaped thermionic source (11) andthe collector electrode (13) are coaxially aligned and closely fitted tothe interior of a passageway defined by a fluid-directing structure(10). An electronic power supply (14) provides a selectably controlledamount of electrical current or voltage on electrical leads (15) tocause a controlled emission of radiant energy from a radiant heatingelement (16) that is directed to the thermionic source (11). The radiantenergy, absorbed by the thermionic source (11), causes the thermionicsource to achieve a predetermined elevated temperature. In response,alkali metal atoms in the interior surface (11B) of the thermionicsource (11) effect a surface of low work function which is capable oftransferring electrical charge while operated at the elevatedtemperature. The collector electrode (13) is electrically connected toan ion current measurement device (17) such as an electrometer which isused to measure the magnitude of ionization current that flows from thethermionic source to the collector electrode (13).

A fluid mixing structure (22) includes a chamber (20) for directing afirst fluid (24), a first detector fluid (26), and a second detectorfluid (27) toward the thermionic source (11). Preferably, the firstfluid comprises a heated, gaseous combination (under pressure) of thesample that is to be analyzed and a carrier gas; the first detectorfluid comprises pressurized hydrogen and (optionally) a make-up gas; andthe second detector fluid comprises air at ambient pressure andtemperature. The second detector fluid (27) is provided via a conduit(29) between the fluid mixing structure (22) and a second detector fluidsource (28).

The aforementioned fluids (24, 26, 27) combine to form a fluid mixture(30) that is restricted to pass through a central bore (11A) defined bythe smooth interior surface (11B) of the thermionic source (11) prior topassing through a second central bore (13A) defined by the smoothinterior surface (13B) of the collector electrode (13). The thermionicsource (11) is termed "flow-through" to denote the use of the centralbore (11A) for substantially all fluid contact of the fluid mixture (30)with the thermionic source (11). In the preferred embodiment, thecentral bore (11A) is cylindrical, such that any erosion of the interiorsurface (11B) will have a nearly negligible effect on the surface areaof the interior surface (11B). Accordingly, the contact of the fluidmixture (30) with the thermionic source (11) is more complete, uniform,and consistent, and less subject to variations in the direction or flowrate of the fluid mixture, than is experienced by thermionic sources inthe prior art. A vent tube (32) allows the further passage of the fluidmixture (30) from the second central bore (13A) to an analyticalinstrument (31), such as a mass spectrometer, that may be optionallyincluded for further analysis of the fluid mixture as known in the art.

FIG. 3 shows a cross-sectional view of a second preferred embodiment ofa thermionic ionization detector (110) constructed according to thepresent invention. A cylindrical flow-through thermionic source (111) iscomposed of a glassy amorphous matrix, preferably fused silica, thatincludes a predetermined concentration of an alkali metal compoundadditive. Alkali metal atoms in the surface layer of the interiorsurface (111S) produce a surface of low work function when operated atan elevated temperature preferably in the range of approximately 100degrees C. to 1000 degrees C. A radiant heater assembly (112) includes aradiant heating element (112E) suspended in an envelope (112V). Theradiant heating element (112E) responds to an applied electrical currentfrom the electronic power supply (14) (see FIG. 2) by emitting radiantenergy, preferably in the infrared region, which heats the thermionicsource (111) to a selected elevated temperature. Preferably, theelevated temperature is selected from temperatures in the range of fromabout 500 degree(s) C. to about 1000 degree(s) C.

The envelope (112V) is formed as a tubular, concentric double-wallstructure composed of an inert material. The envelope (112V) includesinner and outer walls (113A, 113B) that are formed from a single-drawn,fluid-tight structure of fused quartz or fused silica, which istransmissive of infrared radiant energy. ("Fused quartz" and "fusedsilica" are generally equivalent terms that are used herein to refer toan inert, amorphous glassy composition that may include one of manyforms of vitreous silica and the material formed by direct melting ofquartz crystals.) The walls (113A, 13B) are fused together at the bottomof the assembly so as to form a rounded aperture (113D) that defines acentral bore (113R). The walls (113A, 113B) are concentric so as tooffer at least three functions: firstly, to support the heating element(112E) in a manner so as to isolate the element from the corrosiveeffects of the alkali metal or the sample in the fluid mixture;secondly, to maximize the absorption of radiant energy by the thermionicsource (110), and to minimize the loss of radiant energy; and thirdly,to provide an opening (113P) by way of a flange (113F) to allowinstallation or removal of the heating element (112E). The outer wall(113B) is preferably treated or altered so as reflect the radiant energyinward to the thermionic source (111) by, for example, application of anenergy-reflective coating, such as a layer of platinum, silver, ornickel. Low resistance heater element leads (113L) attach to the heatingelement (112E) and exit through a collector body (114) by way of a sidechannel (114C).

The first fluid stream (comprising a mixture of sample, carrier fluid,and make-up fluid)is directed by a jet (115) from a nozzle (115N)positioned coaxially with the thermionic source (111) and the envelope(112V) such that the first fluid stream will combine with a second fluidstream supplied by a supply tube (114T). The combination of the firstand second fluid streams (hereinafter, the fluid stream mixture) entersthe aperture (113P) and then (due to the close fit of the thermionicsource (111) to the inner wall (113A)) is restricted to impinge upon theinterior surface (111S) of the thermionic source (111) before reachingthe collector electrode section (114E) of the collector body (114). Inoperation, the interior surface (111S) of the thermionic source (111) isthereby exposed to substantially all of the sample conveyed by the fluidstream mixture. Sample compounds that are electronegative in chemicalstructure, and therefore readily form gas phase negative ions by theattachment of electrons or negative ions, are ionized by extractingnegative charge from the interior surface (111S) of the source. Thecollector electrode (114E), which is electrically connected by way ofthe collector body (114) to the electrometer (17 shown in FIG. 2),receives an ionization current at the collector electrode (114E).

The radiant heater assembly (112) and collector body (114) areconcentrically clamped onto a detector base (118) by first and secondelectrically insulating sleeves (116A, 16B) that are preferably formedof materials, such as inert polymers, that are creep-resistant and inertto the fluid stream mixture and the elevated temperatures caused by theheater assembly (112). Exemplary inert polymers are polyimides, aramidpolymers, and poly(tetrafluoroethylene) such as are available from theDuPont Company (Wilmington, Del.) under the tradenames Vespel, Kevlar,and Teflon, respectively; and poly(chlorotrifluoroethylene), such asavailable from the 3M Company (Newark, N.J.) under the tradename Kel-F.A gasket (117), preferably formed of a temperature-resistant resilientmaterial such as silicone, provides a fluid-tight joint between thefirst insulating sleeve (116A) and the detector base (118). The detectorbase (118) is attached to a supporting assembly (120) that includes abase sleeve (122) and heated block (124). A capillary column (130)inserted into the jet (115) so as to terminate at a point proximate thenozzle (115N) is preferred for transporting the first fluid stream froma sample inlet (not shown). The capillary column (130) is preferablyformed of synthetic fused silica.

The radiant heating element (112E) includes a resistive filament ofapproximately 0.2 to 0.4 mm platinum (Pt), nichrome (Ni--Cr), oriron-chromium-aluminum (Fe--Cr--Al) alloy formed as a helical coil of adiameter of approximately 6 to 10 mm so as to be positionable betweenthe inner and outer walls (113A, 113B). A preferred wire composition isiron-chromium-aluminum (Fe--Cr--Al) alloy, available as KANTHAL A-1resistance heating alloy, manufactured by the Kanthal Corporation(Bethel, Conn.). The leads (112L) each are preferably formed from anapproximately 20 mil nickel wire having a ceramic insulating sleeve.These dimensions are not to be considered restrictive, and larger orsmaller dimensions can be used with corresponding adjustments in, forexample, the structure of the radiant heater assembly (112) and themagnitude of electrical current supplied to the radiant heating element(112E).

The thermionic source (111) is preferably formed of one or morealkali-metal compounds set in a matrix or substrate. In the preferredembodiment, the thermionic source (111) is composed of a removeable,drawn-glass hollow cylindrical matrix of fused silica or fused quartzthat has been enriched with one or more alkali-metal compounds. Anexample of a suitable fused silica or fused quartz material includesHeraeus powdered quartz, manufactured by Heraeus Amersil Company(Buford, Ga.). An alternative composition would include a hollowcylinder of hardened ceramic material formed from a slurry that includesa mixture of proportionate amounts of water, ceramic cement, and analkali-metal compound. The ceramic cement preferably contains inorganicconstituents such as Al₂ O₃ or SiO₂. An example of a suitable ceramiccement is Ceramacast Type 505 Cement, manufactured by AREMCO Products,Inc. (Ossining, N.Y.)

Generally, the amount and type of alkali-metal compound are selectedaccording to the intended type of surface ionization process sought.Alkali metal compound additives in proportionate amounts ranging from atrace amount by weight to 40% by weight (with respect to theabove-described glassy or ceramic matrix) will exhibit useful ionizationcharacteristics under various operating conditions of the thermionicsource (111). These additives may include compounds of any of the classof alkali metals that includes Cs, Rb, K, Na, and La, and in someinstances may include a combination of more than one type of alkalimetal compound. Specific requirements for the alkali metal compoundsused are that they must have a low volatility at the selectedtemperature of the thermionic source. Alkali sulfate compounds, alkalicarbonates, and alkali chlorides have been found to be suitable. In theglassy matrix, a preferred additive is a Rubidium salt, such as 99.9%Rubidium carbonate (Rb₂ CO₃) or Rubidium Sulfate (Rb₂ SO₄) manufacturedby Alfa/Johnson Matthey, Ward Hill, Mass. In the ceramic matrix,compositions that include Cs₂ SO₄ and ceramic cement will providespecific ionization of sample compounds containing nitrogen orphosphorus atoms.

Any potentially corrosive effects of the fluid flow mixture (30) or thealkali metal in the thermionic source (111) on the heating element(112E) and leads (112L) are thus precluded by the fluid-tight enclosureof the element (112E) and leads (112L) within the envelope (112V),flange (112F), and side channel (114C) in the collector (114).

In the preferred application of the above-described embodiments of athermionic ionization detector, the first fluid flow is taken from theeffluent gas stream of a gas chromatograph instrument. However, thepreferred embodiment of the present invention is not limited inapplication to use as a thermionic ionization detector for a gaschromatograph instrument. Because the contemplated thermionic source(111) provides selective ionization of certain types of chemicalsubstances, this source can also be used in the detection of thepresence of these specific chemical substances in any fluid environment.It is also recognized that the preferred embodiment of the presentinvention can be modified for use as a means of converting molecules ofcertain types of chemical substances into gas phase negative ions forthe purpose of subsequent analysis of charge-to-mass ratio by a massspectrometer instrument, or mass and size analysis by an ion mobilityapparatus. For such applications, the collector (114) would be plumbedto allow the passage of gas phase ions into the subsequent analysisequipment. The possibility of effecting further ion analysis isillustrated diagrammatically in FIG. 2 by the analytical instrument(31).

It is to be recognized that variations and modifications in thethermionic ionization detector might be appropriate for certainapplications and yet be within the scope of this invention. For example,the contemplated radiant heater assembly may be advantageously employedto heat other types and configurations of thermionic sources, such as asimple alkali bead that is fixed in the fluid stream and which does notemploy a "flow-through" configuration. Although the invention has beendescribed with reference to the above-described preferred embodiments,variations and modifications are contemplated as being within the scopeand spirit of the present invention.

What is claimed is:
 1. A thermionic ionization detector for detectingthe presence of a constituent component of a sample in a first fluid,comprising:a thermionic source having a matrix including an alkali metalcompound which is capable of ionization of the constituent component toproduce an ion current when operated at an elevated temperature; aradiant heater having a radiant energy source for emitting radiantenergy, a structure for positioning the radiant energy source relativeto the thermionic source such that the emitted radiant energy issufficient to effect said elevated temperature in the thermionic source;a fluid mixing structure for providing a fluid mixture that includes thefirst fluid to the thermionic source; and a collector electrode forreceiving the ion current.
 2. The apparatus of claim 1, wherein thestructure for positioning the radiant energy source further comprises anenvelope having spaced inner and outer walls, the inner wall defining apassageway for receiving the fluid mixture, the thermionic source beinglocated in the passageway, and the inner and outer walls beingconfigured to retain the radiant energy source therebetween whileproviding fluid-tight isolation between the passageway and the radiantenergy source, and the inner wall being transmissive of the radiantenergy.
 3. The thermionic ionization detector of claim 2, wherein theradiant energy is emitted in the infrared region.
 4. The apparatus ofclaim 3, wherein the outer wall further comprises means for directingthe radiant energy toward the thermionic source.
 5. The apparatus ofclaim 4, wherein the radiant energy directing means further comprises aninfrared reflective coating on the outer wall.
 6. The apparatus of claim1, wherein the elevated temperature effected by the radiant energysource is selected from temperatures in the range of from about 500degree(s) C. to about 1000 degree(s) C.
 7. The apparatus of claim 1,wherein the radiant energy source further comprises a wire filament. 8.The apparatus of claim 7, wherein said wire filament is formed of analloy selected from the following group: platinum (Pt), nichrome(Ni--Cr), and iron-chromium-aluminum (Fe--Cr--Al).
 9. The apparatus ofclaim 7, further comprising means for actuating the radiant energysource for the emission of the radiant energy.
 10. The thermionicionization detector of claim 9, wherein the means for actuating theradiant energy source further comprises an electronic power supply. 11.The apparatus of claim 2, wherein the envelope is formed from anamorphous glassy composition selected from the group consisting ofsilica and quartz.
 12. The apparatus of claim 2, wherein the collectorelectrode and the thermionic source are aligned coaxially within thepassageway.
 13. The apparatus of claim 1, further comprising an ioncurrent measuring device for measuring the ion current received by thecollector electrode.
 14. The apparatus of claim 1, wherein the fluidmixing structure further comprises means for mixing first and seconddetector fluids with the first fluid.
 15. The thermionic ionizationdetector of claim 14, further comprising first and second detector fluidsources, and wherein the first detector fluid comprises hydrogen and thesecond detector fluid comprises air.
 16. A thermionic ionizationdetector for detecting the presence of a constituent component of asample in a first fluid, comprising:a radiant heater having a radiantenergy source for emitting radiant energy, an envelope having spacedinner and outer walls, the inner wall defining a passageway and theinner and outer walls being configured to retain the radiant energysource therebetween while providing fluid-tight isolation between thepassageway and the radiant energy source, and the inner wall beingtransmissive of the radiant energy; a thermionic source positioned inthe passageway and having a matrix including an alkali metal compoundwhich is capable of ionization of the constituent component to producean ion current when operated at an elevated temperature; means fordirecting the radiant energy to the thermionic source; a fluid mixingstructure for providing a flow of the fluid mixture in the passageway,said fluid mixture including the first fluid; and a collector electrodefor receiving the ion current.
 17. A method for detecting the presenceof a constituent component of a sample in a first fluid, comprising thesteps of:providing a thermionic source having a matrix including analkali metal compound which is capable of ionization of the constituentcomponent to produce an ion current when operated at an elevatedtemperature; directing radiant energy from a radiant heater having aradiant energy source to the thermionic source, the radiant energy beingsufficient to effect said elevated temperature in the thermionic source;providing a fluid mixture flow that includes the first fluid to thethermionic source; receiving the ion current at a collector electrode;and measuring the received ion current and, in response, indicating thepresence of the constituent component.